In a primary switched-mode power supply which is commercially available, the output of said primary switched power supply is not protected by means of overvoltage or overcurrent sensitive components. Said power supply is protected only against short-circuits, since in case of a short-circuit the in-phase winding of the auxiliary power supply of the control unit is also shorted by means of the magnetic mutual coupling. Thus, in case of a short circuit, the control unit is turned off after a predetermined time. Upon occurrence of an overcurrent, the auxiliary power supply remains on and thus, by virtue of the magnetic mutual coupling of the in-phase winding with the secondary circuit, the control unit remains on, so that in the primary winding circuitry the primary power duty cycle is increased, instead of being reduced or turned off as it should be.